There are noticeable differences in perception of audiometric and ultrasonic signals by human beings. The resolution of human hearing, for example, is measured by a quantity referred to as the Just Noticeable Difference (JND). This parameter is determined experimentally as follows. A subject listens to a tone generated at a certain sound pressure level. The frequency of the tone is then shifted slightly and the JND for that frequency and sound pressure level is the amount of frequency shift which can be perceived by the subject.
Using the JND technique, it has been determined that human hearing operates on a logarithmic scale, so that the resolution at low frequencies is finer, in an absolute sense, than at higher frequencies. Generally, the JND is about 1.4% of the test frequency averaged over different sound pressure levels. For example, a 1 KHz test tone yields a JND of about 14 Hz. The same logarithmic behavior is evident in ultrasonic hearing, but with a larger conversion factor on the order of 12%.
The present invention relates to a method and apparatus for translating audiometric signals into the ultrasonic range. The translated signals can be delivered to an ultrasonic transducer which when properly placed on an individual allows the individual to perceive the ultrasonic signals as audible sound.